It is clear from many studies performed in the U.S. and abroad that there is an urgent need to develop a vaccine which can be used in both industrialized and developing countries. In an attempt to achieve this goal we have continued our laboratory and field studies of vaccine candidates which include rhesus rotavirus (RRV) strain MMU18006 and reassortants derived from it. RRV is a simian rotavirus strain derived from a stool of a 3.5 month old rhesus monkey with acute diarrhea. It has been passaged 9 times in primary or secondary monkey kidney cell culture and 7 times in DBS- FRhL-2 cell culture, a semi-continuous simian diploid cell strain. The rhesus rotavirus (RRV) appears to be restricted in humans, and its major neutralization protein (VP7) is very closely related antigenically to the corresponding protein of human rotavirus serotype 3. Reassortant rotaviruses with 10 genes from RRV and a VP7 gene from serotype 1, serotype 2 or serotype 4 human rotavirus were prepared in LID and are now being evaluated in phase 1 clinical trials. In phase I and phase II (efficacy) studies in Venezuelan infants the RRV vaccine was shown to be satisfactorily immunogenic and protective. RRV reassortants have also been tested singly or in combination or in combination with RRV in an attempt to induce heterotypic immunity capable of providing resistance to the four commonly circulating rotavirus serotypes.